[release/7.0] [metadata] Skip null vtable entries when checking covariant return overrides

src/mono/mono/metadata/class-setup-vtable.c

@@ -1656,6 +1656,10 @@ check_vtable_covariant_override_impls (MonoClass *klass, MonoMethod **vtable, in
 				break;
 			MonoMethod *prev_impl = cur_class->vtable[slot];
 
+			// if the current class re-abstracted the method, it may not be there.
+			if (!prev_impl)
+				continue;
+
 			if (prev_impl != last_checked_prev_override) {
 				/*
 				 * the new impl should be subsumed by the prior one, ie this

src/tests/Loader/classloader/MethodImpl/CovariantReturns/UnitTest/OverrideReabstracted.cs

@@ -0,0 +1,52 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+
+
+namespace ReproMAUI6811;
+
+public static class Program
+{
+    public static int Main()
+    {
+        Leaf l = new Leaf();
+
+        if (l.getI().ToString() != "Leaf")
+                return 1;
+        if (((Intermediate)l).getI().ToString() != "Leaf")
+                return 2;
+        if (((PseudoBase)l).getI().ToString() != "Leaf")
+                return 3;
+        if (((Base)l).getI().ToString() != "Leaf")
+                return 4;
+        return 100;
+    }
+}
+
+public abstract class Base {
+    public abstract I getI();
+}
+
+public class PseudoBase : Base {
+    public override I getI() => new C ("PseudoBase");
+}
+
+public abstract class Intermediate : PseudoBase {
+    public override abstract I getI();
+}
+
+public class Leaf : Intermediate {
+    public Leaf() {}
+    public override C getI() { return new C ("Leaf"); }
+}
+
+public interface I {}
+
+public class C : I {
+    private readonly string _repr;
+    public C(string s) { _repr = s; }
+    public override string ToString() => _repr;
+}
+
+

src/tests/Loader/classloader/MethodImpl/CovariantReturns/UnitTest/OverrideReabstracted.csproj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>